The British domestic intelligence agency MI5 has alerted aviation and police officials that al-Qaeda may be planning a terrorist attack against a U. K. airport or other British target in a threat described as credible, according to sources at the New Scotland Yard and the British Home Office.

The intelligence officials added that no changes to security will be made and the planned method of attack is still unclear. According to the Scotland Yard source, British counterterrorism officials immediately warned their U.S. counterparts of a possible terror threat to planes flying through the U.K.

There are currently several U.S. counterterrorism teams posted in London and other parts of the U.K., including a team of New York City police detectives from the NYPD's intelligence division.

The NYPD officers assigned in London would not give details, but indicated that British authorities have said they were following the mandated protocols in the U.K. The heightened alert came as a result of increased "chatter" among suspects under surveillance by British counterterrorists, but there is no imminent threat that no threat.

The Metropolitan Police public information officer stated that the overall threat level from international terrorism remains at "severe" — the second-highest level, meaning an attack is highly likely. The level has not changed since January 2010.

The Home Office stated that any imminent, serious threat to public safety would prompt a change in the overall threat level — and would see it raised to "critical," the highest point on the system's five point scale.

According to MI5, a significant number of British nationals and foreign citizens who reside in the UK are known to be linked to or sympathetic with Al Qaeda.

They are supporting the activities of terrorist groups in a range of ways. For example, they are:

acquiring and disseminating false documents for use by terrorists in the UK and overseas; and

facilitating training in the UK and overseas in extremist ideology and terrorist techniques.

In some cases they have also been engaged in directly planning, or attempting to carry out, terrorist attacks. Some British citizens and residents have received terrorist training in camps overseas. Relationships forged in these training camps have formed the basis of loose networks of terrorists who can operate outside structured organizations. Others have formed groups within the UK in which members reinforce each others' adherence to extremist ideologies.

A number of British residents have traveled to Iraq and Afghanistan to join the insurgencies against those countries' governments and international forces. Some have returned to the UK and are believed to be involved in supporting and planning terrorist activities here.

More than 40 terrorists have been convicted under the Terrorism Act and another 183 have been convicted of terrorist-related offenses, including murder, illegal possession of firearms and explosives offences. 1,165 people have been arrested under the Terrorism Act and 114 are awaiting trial.

"International terrorism is a nationwide problem. Those involved in international terrorism are not associated with any single area of the UK; individuals convicted of terrorist offences have lived throughout the country. Likewise, terrorists have sought to target a variety of different places. Attacks related to international terrorism have occurred in London and Glasgow, and thwarted terrorist plots have been aimed at targets outside the capital," according to a statement from London's Metropolitan Police.